Metastable Carbon at Extreme Conditions

TL;DR

This study investigates the stability of various carbon phases under extreme pressures, revealing diamond polytypes as the only metastable forms at high pressures relevant to shock experiments and planetary interiors.

Contribution

It identifies diamond polytypes as the only metastable carbon phases energetically competitive with cubic diamond between 100 and 1,000 GPa, expanding understanding of carbon's behavior under extreme conditions.

Findings

Diamond polytypes are the only metastable phases between 100 and 1,000 GPa.

No metastable phases are competitive above 1 TPa with stable BC8 and simple cubic phases.

Low enthalpy diamond polytypes may appear during shockwave loading of diamond.

Abstract

Carbon at extreme conditions is the focus of intensive scientific inquiry due to its importance for applications in inertial confinement fusion experiments and for understanding the interior structure of carbon-rich exoplanets. The extreme metastability of diamond at very high pressures has been discovered in recent dynamic compression experiments. This work addresses an important question about the existence of other competitive metastable carbon phases that might be observed in shock experiments. It was found that diamond polytypes, carbon crystals with mixed cubic and hexagonal diamond stacking planes, are the only metastable carbon crystal phases energetically competitive with cubic diamond at pressures between 100 and 1,000 GPa. Above 1 TPa, no metastable phases are found to be energetically competitive with thermodynamically stable BC8 and simple cubic phases. The existence of low enthalpy diamond polytypes suggests that they are likely candidates for metastable phases of carbon to appear upon shockwave loading of diamond.